You could fit in any villain you like in place of Khan
Actually it isn't as easy as people seem to think as with the villains suggested as replacements are either to under powered to slaughter a platoon of klingons are to over powered for them not to just vaporize the klingons with a thought.
Seeing Gary-Cumberbatch vaoprize a platoon of klingons with his telekineitc and telepathic powers would have been awesome to see though.
But I agree. Without nerfing Gary Mitchell to a level that i can be beaten, there would be no way to maintain him as a threat throughout the film.